The invention relates to a drive train comprising an internal combustion engine and two electric drive units, and to a method for operating the same. Furthermore, the invention relates to a group of drive trains.
DE 196 06 771 C2 discloses a hybrid drive for motor vehicles, which has an internal combustion engine and two electric drive units. A first power branch is provided between an engine shaft of the internal combustion engine and an output element, such as an output shaft. The drive torque of the internal combustion engine passes via said power branch. The engine shaft can be coupled directly to the output shaft via a clutch. The first electric drive unit exchanges power with the first power branch in such a way that the drive torque of the first electric drive unit can be superimposed on the drive torque of the internal combustion engine. The second power branch is operated by the second electric drive unit. The power of the second drive unit is superimposed on the power of the first power branch and that of the second power branch, that is to say the power of the internal combustion engine and, if appropriate, the power of the first electric drive unit, via a pick off gear unit, such as a planet gear unit. According to the prior art, a sun gear of the pick off gear unit is connected to the engine shaft, while the torque of the second electric drive unit can optionally be transferred to two ring gears of the pick off gear unit with different diameters via two clutches of the second power branch. A speed-transforming transmission is connected between the clutches and the ring gears.
Further known hybrid drives are described in, for example, DE 41 24 479 C2, WO 94/19856 and DE 199 16 489 C2.
The present invention is based on the object of proposing a drive train and a method for operating the same which are improved in terms of combination with existing transmission concepts,
different operating ranges,
different operating strategies,
power and/or torque balances and/or
formation of groups with different drive train concepts.
Furthermore, the object of the invention is to propose a group of drive trains which ensures a modular design for subgroups of different drive trains with a high number of identical parts.
The object on which the invention is based is achieved in that at least two operating positions are provided in the drive train. In a first operating position, the above-mentioned power branches can be coupled directly to one another, for example by a suitable clutch. Therefore, in this first operating position, the torque of the second electric drive unit is directly superimposed on the drive torque of the internal combustion engine without intermediate connection of further speed-transforming transmissions. In this operating position, a torque of the second drive unit can thus be added to the torque of the internal combustion engine, operation can take place exclusively by means of the second electric drive unit, energy can be recovered, for example by being fed into a battery or for operating the first electric drive unit, and/or the internal combustion engine can be operated by the second electric drive unit during a start operation.
According to the invention, in a second operating position the two power branches can be coupled to one another via the pick off gear unit. In this context, the two power branches each have a drive connection to two transmission elements (the transmission elements of the ring gear, planet, web, sun gear) of the pick off gear unit. The output of the pick off gear unit is formed in this case by a third transmission element. The superimposition by the pick off gear unit results in a variable transmission in the direction of the output element. In this way it is possible to implement, for example, what is referred to as a geared neutral point, represented by the output element when the internal combustion engine is working and the second electric drive unit is working. Furthermore, a forward or backward movement of the output element is possible depending on the rotational speed of the second electric drive unit.
The inventive configuration results in a configuration of the transmission ratios and operating possibilities which is improved compared to the prior art together with an optimum utilization of installation space.
According to a further proposal of the invention, the transmission element which conducts the drive torque of the internal combustion engine and, if appropriate, the drive torque of the second electric drive unit has a drive connection to a ring gear of the pick off gear unit. If the output is provided via a further transmission element of the pick off gear unit, the drive torque of the transmission element can easily be transmitted in the direction of the output element.
The pick off gear unit preferably has a double planet gear which has a drive connection to the abovementioned ring gear and to a second ring gear. The number of possible operating modes of the drive train can be increased by the second ring gear.
A particularly advantageous inventive configuration of the drive train is obtained if the output element is an input element of a component transmission which is connected downstream. This permits the inventive measures to be combined, for example in accordance with DE 199 10 299 C1, with a component transmission which is known. As a result, advantages of the measures according to the invention can be combined with advantages of transmissions which are known. The configuration according to the invention permits, in contrast with the above-mentioned prior art, the motor vehicle to be driven purely electrically while using all the gear speeds of the component transmission which is connected downstream. Furthermore, the component transmissions which are connected downstream can be used on the one hand for combination with an internal combustion engine and two electric drive units in drive trains, that is to say for a hybrid drive, and also likewise for other drive trains which only have, for example, one internal combustion engine. This permits the component transmission to be manufactured in large numbers for different purposes of use with a high proportion of identical components.
In a method according to the invention for operating a drive train, the drive train is operated in accordance with the operating conditions. The operating conditions are, for example, an operating temperature or operating period of the drive train, an operating temperature or operating frequency or a wear state of a starting element, a clutch or a brake, a charge state of a battery, a detected driving environment or a driver's request.
Depending on the operating conditions, the internal combustion engine is started (in some cases) with the second electric drive unit switched off and clutches opened, by applying the output torque of the first electric drive unit to the internal combustion engine. This permits, in particular, warm starting of the internal combustion engine by means of the first electric drive unit. For different operating conditions, the internal combustion engine is started with clutches closed, by applying the output torque of the first electric drive unit and of the second electric drive unit to the internal combustion engine. The torque which is available to crank the internal combustion engine is thus obtained by superimposing the drive torques of the two drive units. Owing to the increased available torque, this operating mode is suitable in particular for cold starting the internal combustion engine. For both of the abovementioned operating modes, the electric drive units are fed by a vehicle battery. Furthermore it is possible to crank the internal combustion engine with the first electric drive unit while the starting clutch is opened, while at the same time the motor vehicle is driven electrically by the second electric drive unit.
According to a development of the method according to the invention, depending on the operating conditions, the internal combustion engine is started with clutches closed, by applying the drive torque of the first electric drive unit and of the second electric drive unit to the internal combustion engine, with the pick off gear unit being intermediately connected in this case between the second electric drive unit and the internal combustion engine. This makes it possible for the output torque of the second electric drive unit to be transmitted in the direction of the internal combustion engine, permitting a further increased drag torque for the internal combustion engine. This operating mode is particularly suitable for extreme starting of the internal combustion engine.
According to developments of the method according to the invention, different partial operating ranges for the flow of force to the output element are possible during a driving mode:                In partial operating ranges with maximum power demand, drive is provided by the internal combustion engine, the first electric drive unit and the second electric drive unit.        In partial operating ranges in which there is neither an increased power demand nor is there a need for energy to be recuperated by the electric drive units, drive is provided exclusively by the internal combustion engine.        When drive is being provided by the internal combustion engine, the energy is fed back into a battery by the first and/or second electric drive unit in order to recover energy, for example in order to recharge the battery.        In partial operating ranges it is also possible for drive to be provided by the internal combustion engine and an electric drive unit, in particular the first electric drive unit. This electric drive unit is fed at least partially by the other electric drive unit which is used in the generator operating mode, in particular by the second electric drive unit. This saves the battery and/or permits longer operation when the battery is used.        
According to a further proposal for achieving the object on which the invention is based, a group of drive trains is provided which each have a transmission connected downstream of the output element, for example in accordance with DE 199 10 299 C1.
This group has different subgroups of drive trains of different designs.                There is a first subgroup of drive trains which have a hybrid drive with an internal combustion engine and two electric drive units.        For a second subgroup of the drive trains, a hydrodynamic torque converter, instead of the parts which are necessary for the hybrid drive, is connected between the internal combustion engine and the output element in the installation area of the first and/or second electric drive unit (instead of said unit or units).        
This embodiment of the invention results in a modular design for which the component transmission which is connected downstream of the output element can be combined with a suitable front mounted module so that when the component transmissions are the same a hybrid drive and a conventional drive, in particular an automatic transmission with a hydrodynamic torque converter, can easily be provided with a large number of identical components.
According to one inventive development of the group of drive trains, there is a third subgroup for which a starting clutch is intermediately connected between the internal combustion engine and the output element in the installation area of the first and/or second electric drive unit (instead of said drive unit or units). Accordingly, three different variants of drive trains can be provided with a simple modular design. For example, the third subgroup comprises drive trains for implementing an automatic transmission with an operating behavior which is more sporty compared to the second subgroup.
One particularly advantageous group of drive trains is obtained if the pick off gear unit is an input-end planet set of the component transmission. As a result it is possible to further reduce the number of necessary components needed for implementing a hybrid drive in accordance with the first subgroup.